1. Field of the Invention
The invention relates to a disc-shaped recording medium to record or reproduce, for instance, digital audio signals.
2. Description of the Prior Art
What is called a compact disc (CD) which audio signals have been converted into digital signals and recorded onto an optical disc is widespread. FIG. 1 shows a construction of an optical disc 101 which is used as a conventional compact disc. In the conventional compact disc, for instance, an outer diameter is set to 12 cm and, as shown in FIG. 1, a lead-in area 102 is provided in the innermost rim and a lead-out area 103 is provided in the outermost rim. A region between the lead-in area 102 and the lead-out area 103 is used as a data area 104. A TOC (Table Of Contents) to record recording position information of data is provided in the lead-in area 102. A predetermined pattern is recorded in the lead-out area 103. Music data or the like is recorded in the data area 104. The music data is what is called straight PCM audio data which is obtained by simply linearly quantizing analog audio signals. In such a conventional compact disc, data is recorded as pits in all of the lead-in area 102, lead-out area 103, and data area 104.
In a disc reproducing apparatus for reproducing such a conventional compact disc, when a disc is loaded, the lead-in area 102 is first accessed and the information of the TOC in the lead-in area 102 is read. A desired data position is accessed by using the information of the TOC and the recording data is reproduced. When a pickup reaches the lead-out area 103 of the outermost rim, a desired pattern which is recorded into the lead-out area 103 is reproduced. Due to this, it is detected that the pickup has reached the outermost rim of the optical disc 101.
The above conventional compact disc is of the read only type and data cannot be recorded. Therefore, a rerecordable write once type CD-WO or a recordable and reproducible CD-MO using a magneto-optic disc has been proposed. In a magneto-optic disc 201 as a recordable and reproducible CD-MO, as shown in FIG. 2, a portion between a lead-in area 202 and a lead-out area 203 is used as a data area 204. TOC data is recorded into the lead-in area 202 by pits. A predetermined pattern is recorded into the lead-out area 203 by pits. Desired data can be recorded or reproduced into/from the data area 204 by marks in the magnetizing direction of a perpendicular magnetization film.
In such a CD-MO, the lead-in area 202 and lead-out area 203 are set to the areas (hereinafter, referred to as premastered areas) in which data is recorded by pits. The data area 204 is set to the area (hereinafter, referred to as a magnetooptic area) in which data is recorded by marks in the magnetizing direction of the perpendicular magnetization film. Therefore, it is necessary to switch the reproduction signals from the pickup. Namely, the reproduction signal of the premastered area is obtained from the level of the reflected light from the disc. The reproduction signal of the magneto-optic area is obtained by detecting the direction of a Kerr rotational angle. That is, the reproduction signal of the premastered area is derived from the sum signal of the signals from two photodetectors of the pickup. The reproduction signal of the magneto-optic area is obtained from the differential signal of the signals from the two photodetectors of the pickup. Therefore, in the lead-in area 202 and lead-out area 203 comprising the premastered areas, the sum signal of the signals from the two photodetectors of the pickup needs to be generated. In the data area 204 comprising the magneto-optic area, the differential signal of the signals from the two photodetectors of the pickup needs to be generated.
FIG. 3 shows an example of a switching circuit of the reproduction signals. In FIG. 3, outputs RF1 and RF2 of the two photodetectors of the pickup are respectively supplied to input terminals 251 and 252. The reproduction signals RF1 and RF2 from the input terminals 251 and 252 are supplied to an adding circuit 253 and are also supplied to a subtracting circuit 256. The outputs RF1 and RF2 of the two photodetectors are added by the adding circuit 253. The subtraction between the outputs RF1 and RF2 of the two photodetectors is executed by the subtracting circuit 256. An output SRF of the adding circuit 253 is supplied to a terminal 254A of a switching circuit 254. An output SMO of the subtracting circuit 256 is supplied to a terminal 254B of the switching circuit 254. An output of the switching circuit 254 is generated from an output terminal 255.
When the premastered areas are being reproduced, the phases of the outputs RF1 and RF2 of the two photodetectors are equal. When the outputs RF1 and RF2 of the two photodetectors are added by the adding circuit 253, the reproduction signal of the physical pits is obtained. The output which is obtained by executing the subtraction between the outputs RF1 and RF2 of the two photodetectors by the subtracting circuit 256 discard.
When the magneto-optic area is reproduced, the phases of the outputs RF1 and RF2 of the two photodetectors are opposite. By performing the subtraction between the outputs RF1 and RF2 of the two photodetectors by the subtracting circuit 256, the reproduction signal of the marks in the magnetizing direction of the perpendicular magnetization film is derived. The output which is obtained by adding the outputs RF1 and RF2 of the two photodetectors by the adding circuit 255 is discard.
In FIG. 3, when the lead-in area 202 and the lead-out area 203 as premastered areas are reproduced, the switching circuit 254 is set to the terminal 254A side. Due to this, the data of the lead-in area 202 and the data of the lead-out area 203 are reproduced from the sum signal of the outputs RF1 and RF2 of the two photodetectors. When the data area 204 as a magneto-optic area is reproduced, the switching circuit 254 is set to the terminal 254B side. Consequently, the data recorded in the data area is reproduced.
As mentioned above, in the conventional CD-MO disc the lead-out area 203 is set to the premastered area. However, when the lead-out area 203 is formed by pits, a state in which the pickup has reached the outermost rim of the disc cannot be detected even when the pickup has been moved from the magneto-optical area to the lead-out area 203 and there is a danger such that the servo runs away because the recording data is not correctly reproduced.
That is, when the data area is reproduced, the switch 254 in FIG. 3 is connected to the terminal 254B side and a difference signal between the outputs RF1 and RF2 of the two photodetectors is generated. Assuming that the lead-out area 203 has been formed by pits, when the pickup reaches the lead-out area 203 of the outermost rim, unless the switching circuit 254 is switched to the terminal 254A side, the data in the lead-out area 203 cannot be reproduced. Therefore, unless the switching circuit 254 is correctly switched, even when the pickup reaches the lead-out area 203 of the outermost rim, a predetermined pattern of the lead-out area 203 cannot be reproduced and there is a possibility such that the pickup is deviated out of the position of the disc.